Some offshore rigs used for drilling oil or gas wells move relative to the sea floor as a result of sea currents and wave motion. This movement can make landing a casing string in the well bore a difficult task. As the drilling rig moves in an up and down motion, the string of casing pipe moves relative to the well bore. If the casing shoe hits the bottom of the well bore before the casing string is properly landed in the casing hanger this can cause the casing string to become damaged. Devices known as motion compensators have been developed to control this movement. These devices utilize a combination of cables, pulleys and hydraulic cylinders. Although motion compensators dampen a significant portion of the movement of the drilling rig, they cannot completely control all of the motion imparted to the drilling rig.
One solution to this problem is to land the casing string a given distance above the bottom of the well bore, typically 15 feet, and cement the casing string in this position before drilling the next section of the well bore. This allows the casing string to move axially relative to the well bore as the drilling rig moves in an up and down motion without the casing shoe hitting the bottom of the well bore before the casing string is properly landed in the casing hanger.
The well bore between the bottom of the casing string and the bottom of the well is sometimes called the rat hole and is of a larger diameter than the outside diameter of the casing being installed. After the casing string has been cemented in place, the next section of the well bore is drilled. Typically, this next section is smaller in diameter than the previously drilled section.
While this solution protects the casing string, as it is being landed, from damage due to the up and down movement of the drilling rig, it is not without drawbacks. When logging and other tools are lowered into the well bore to evaluate the subsea formation conditions in the newly drilled section of the well, they tend to get stuck or "hung up" in the larger diameter rat hole. This is especially likely to occur in deviated wells, that is wells which are drilled at some angle less than 90 degrees to the surface of the sea floor. The greater the angle of incline the greater the possibility that the logging or other tools will get stuck. Once stuck, it is very difficult to recover a tool without damaging it, and sometimes it is impossible to recover the tool at all. Replacement and/or repair of logging and other exploratory tools can be very costly because these devices are very expensive pieces of equipment. Also, the recovery procedure can cause damage to the well which itself may require repair.
Furthermore, the rat hole has a tendency of filling up with formation cuttings and cement chunks. These fillings can cause the drill bit of the drilling tool to get stuck in the rat hole thereby impeding the process of drilling the lower section of the well bore.
The present invention is directed to overcoming or at least minimizing some of the problems mentioned above.